Styrene-based resins are used in an extremely wide range of applications as a result of not only having superior material performance in terms of transparency, rigidity, dimensional stability and the like, but also as a result of being able to be processed in various forms such as rolled sheets, films, foamed sheets, foamed boards and blow molded articles, and because many styrene-based resins can be produced inexpensively and in large volume by bulk polymerization using radical polymerization, solution polymerization using a high monomer concentration, suspension polymerization or emulsion polymerization.
Although typical examples of styrene-based resins may include polystyrene (GPPS), styrene/acrylonitrile (AS), styrene/methyl methacrylate (MS), styrene/methacrylic acid (SMM), styrene/maleic anhydride (SMA) and the like, styrene homopolymers (polystyrene (GPPS)) is used the most universally.
Polystyrene has numerous superior properties, and because it is inexpensive, has high usage value and is used in various applications. Some major applications thereof are indicated below.
(Packaging Applications)
Lunchbox containers (foamed sheet: PSP), cup noodle containers (foamed sheet: PSP), clear cups, spoons, forks, vegetable packaging sheets (biaxially oriented sheets), envelop windows
(Home Appliance Applications)
Television, air-conditioner and OA equipment packaging, electric refrigerator trays, cassette, MD and MO shells
(Miscellaneous Household Goods)
Toys, stationary articles
(Building Material Applications)
Insulation (foamed boards), tatami mats (foamed boards)
However, there were some applications that were unable to be satisfied even by the performance of this resin, such as applications that prevented this resin from being used due to insufficient heat resistance. More specifically, since the GPPS heat resistance is about 100° C. (glass transition temperature), in applications involving contact with hot steam for disinfection by boiling, food packaging applications requiring heating in a microwave oven or applications involving molded automotive parts susceptible to exposure to high temperatures in the summer, this resin was unable to be used without risk of causing deformation to the molded articles.
One way of enhancing the heat resistance of polystyrene may include copolymerizing styrene with a monomer containing a polar functional group, examples of which may include copolymers of styrene and methacrylic acid (SMAA), copolymers of styrene and maleic anhydride (SMA) and copolymers of styrene and maleimide anhydride. Heat resistance can be arbitrarily changed by controlling the amount of monomer containing the polar functional group in the copolymer composition. For example, a typical example of a heat-resistant styrene-based resin in the form of SMAA has a Vicat heat resistance temperature of from 105 to 125° C. However, when polymers having the polar functional group are exposed to high temperatures, a crosslinking reaction of the polymer chain occurs due to side reactions of the polar groups, and as a result, gel-like substances are formed that cause a decrease in molding processability due to increased viscosity, thereby preventing these polymers from being adequately accepted by users from the viewpoints of quality and productivity.
In addition, increased susceptible to the occurrence of crosslinking reactions under conditions of high-temperature melt retention means increased susceptibility of high molecular weight polymers to deformation during molding processing, thereby making it difficult to recycle and reuse resins. For example, when obtaining an injection molded article, end materials (skeletons) are generated during formation of sprue and runner components and when obtaining molded articles from biaxially oriented sheets or foamed sheets. These are usually reused by partially mixing with virgin pellets after being crushed or cut up, or are typically reused by partially mixing into general-purpose resins such as polystyrene.
However, reuse become difficult if the flow properties of a resin change due to crosslinking of high molecular weight polymers during melt processing, and there was the problem of limitations being placed on the use of these resins for recycling to virgin pellets. Moreover, copolymers containing the polar functional group typically have poor compatibility with polystyrene and even if mixed by melting, not only do they lead to a decrease in mechanical properties, but also cause a loss of transparency, thereby preventing these copolymers from being recycled to general-purpose polystyrene.
With the increasing emphasis being placed on effective utilization of resins in recent years, various types of recycling methods have been developed and implemented. Being able to recycle, rework and reuse resins is an essential requirement on the resin markets of the future. Resin materials developed in the future will be required to be resins able to be effectively recycled without undergoing hardly any decrease in molecular weight or generation of monomers due to severing of the polymer chain even if going through several rounds of melt processing. Thus, there is a desire for the development of resin materials exhibiting higher melt stability than conventional styrene-based copolymers.
Another problem of conventional heat-resistant styrene-based resins was the narrow range of processing conditions during molding. Improving the heat resistance of a copolymer is equivalent to improving the temperature at which flow of the polymer chain begins. Thus, if it is desired to obtain flow properties similar to those of polystyrene during molding processing, it is necessary to raise the processing temperature corresponding to an improvement in heat resistance. However, in the case of styrene-based copolymers containing the polar functional group, the decomposition starting temperature does not improve corresponding to heat resistance. Consequently, the molding processing temperature range becomes narrow and as a result, there were problems in terms of leading to decreases in productivity and quality.
There are also methods for improving the heat resistance of styrene-based resins by using a monomer that does not contain the polar functional group. For example, a copolymer of styrene and α-methylstyrene is known to demonstrate an increase in the glass transition temperature as the content of α-methylstyrene increases (see, for example, Non-Patent Document 1).
However, in the case of attempting to copolymerize styrene and α-methylstyrene using a typical example of an industrial production process in the form of radical solution polymerization, none of these methods have been able to be used industrially due to numerous problems as indicated below:    1) high molecular weight polymerization is difficult due to the low ceiling temperature of α-methylstyrene of about 60° C.;    2) target heat resistance cannot be obtained due to limitations on the content of α-methylstyrene in the copolymer; and    3) thermal decomposition of the copolymer occurs easily depending on molding processing conditions due to poor thermal stability during melting, thereby resulting in the generation of monomer components and susceptibility to decreases in molecular weight.
On the other hand, since α-methylstyrene can undergo living anionic polymerization by using a butyl lithium initiator, copolymers of styrene and α-methylstyrene can be also be produced by living anionic polymerization (see, for example, Patent Document 1).
Therefore, a method has been proposed for solving the problems of radical solution polymerization by carrying out living anionic polymerization according to a continuous living polymerization method using a complete mixing type of polymerization reactor (see, for example, Patent Document 2). This method is characterized by carrying out polymerization using a continuous type of complete mixing reactor such that the concentrations of α-methylstyrene monomer, styrene monomer and living copolymer present in the living polymerization reaction system remain constant at all times, and offers the following effects:    1) high molecular polymerization is possible due to the use of a living polymerization method;    2) the content of α-methylstyrene in the copolymer can be controlled since the concentration in the reaction system can be kept constant; and,    3) thermal stability during melting improves due to the absence of head-to-head bonds, tail-to-tail bonds and other unstable bonds in the main chain of copolymer due to the use of anionic polymerization.
However, living anionic polymerization is typically susceptible to the effects of impurities contained in the raw materials. In particular, active anions are known to easily react with polar substances such as water, aldehydes, ketones and alcohols. If polar substances are present, even in trace amounts, in an anionic polymerization reaction system, the active anions react with the polar substances resulting in the formation of stable bonds, thereby causing the problem of interrupting polymerization. Consequently, when carrying out living anionic polymerization, polar substances in raw materials must be reduced, and entrance of polar substances into the reaction system must be suppressed as much as possible.
In addition, the above-mentioned polar substances and reaction products of active anions and polar substances undergo degeneration during the course of the polymerization process resulting in the possibility of the formation of colored substances. In this case, this can result in coloring of the polymer or decreases in polymer physical properties, thereby making this undesirable. From this viewpoint as well, polar substances in the raw materials are preferably eliminated as much as possible.
However, the typical process for industrial production of α-methylstyrene is the cumene-phenol production process. This cumene-phenol production process contains a step in which cumene hydroperoxide obtained by oxidation of raw material cumene is concentrated to 80 to 85% followed by acid decomposition to phenol and acetone and neutralization/washing. The resulting crude phenol contains such substances as acetone, water, cumene, α-methylstyrene and phenol, and purified phenol is produced from this crude phenol by distillation, while at the same time, acetone and α-methylstyrene are respectively separated and recovered (see, for example, Patent Documents 3 and 4).
However, in the case of attempting to respectively separate and recover acetone and α-methylstyrene simultaneous to production of purified phenol using the process described above, the following problems occur. Namely, substances having a boiling point close to that of α-methylstyrene for which separation is difficult by distillation are present among the polar substances such as aldehydes and ketones produced as by-products during phenol production. Thus, polar substances end up contaminating the α-methylstyrene in the form of impurities in the case of distillative purification alone.
In addition, a method is carried out for the purpose of removing impurities containing polar substances comprises supplying crude α-methylstyrene to an alkaline washing tank prior to distillation and washing in the tank followed by supplying to a distillation column to recover the product α-methylstyrene (see, for example, Patent Documents 5 and 6). However, in the case of washing in an alkaline washing tank according to the above method, since the solubility of polar substances in alkaline solution is inadequate, the polar substances end up remaining in the α-methylstyrene, thereby preventing the removal of trace amounts of polar substances. In addition, although methods have also been considered involving reacting polar substances in an alkaline washing tank to obtain lowly volatile substances and increasing the difference in relative volatility between these substances and α-methylstyrene followed by distillation, in this case, polar substances cannot be removed unless the reaction is allowed to proceed to nearly 100%.
On the other hand, an example of a typical method for purifying polymerization monomers in the form of styrenes in the laboratory may include washing with an alkaline substance such as an aqueous sodium thiosulfate solution or aqueous sodium hydroxide solution and water followed by drying and distillation (see, for example, Non-Patent Document 2). However, although the above method makes it possible to eliminate the use of a polymerization inhibitor, peroxide and polymer, polar substances cannot be completely removed from α-methylstyrene for the same reasons as in the case of washing in an alkaline washing tank as described above.
In addition, although a method has indicated including adding alkyl lithium and the like to monomers prior to polymerization to deactivate polar substances followed by distillation, in this case, since considerable amounts of polymers, oligomers and the like are formed accompanying the reaction, this method is not considered to be industrially suitable. In addition, the monomers may become contaminated with oligomers depending on the distillation conditions, thereby having the potential for having a detrimental effect on polymerization.
Another example of a method for purifying α-methylstyrene may include purification using a column packed with silica gel, alumina or ion exchange resin and the like (see, for example, Patent Document 2 and Non-Patent Document 3). However, these packing materials usually contain acidic or basic components. Namely, in the above method, there is a possibility of an acidic component present in the packing material causing the formation of low molecular weight oligomers of α-methylstyrene, or a basic component present in the packing material causing deterioration of polar substances to high molecular weight condensates. The formed oligomers or high molecular weight condensates contaminate the monomers in the column, and if polymerization is carried out using these contaminated monomers, there is a risk of polymerization being interrupted or the reaction being impaired in other ways. Moreover, since these oligomers and high molecular weight condensates have low volatility, they are unable to be removed from the polymerized polymer solution, and end up contaminating the final product polymer. As a result, problems occur leading to deterioration of polymer performance in the form of a decrease in the heat resistance of the product polymer or yellowing.
As has been described above, none of the methods of the prior art are able to be effectively used industrially as a method for purifying α-methylstyrene.
Patent Document 1: Japanese Patent Publication No. H6-10219
Patent Document 2: Japanese Patent Application Laid-open No. 2006-052346
Patent Document 3: Japanese Patent Application Laid-open No. S55-94326
Patent Document 4: Japanese Patent Publication No. S64-7058
Patent Document 5: Japanese Patent Application Laid-open No. 2000-86559
Patent Document 6: Japanese Patent Application Laid-open No. H3-258733
Non-Patent Document 1: Journal of Applied Polymer Science, Vol. 41, p. 383 (1990)
Non-Patent Document 2: R. H. Boundry, R. F. Boyer, “Styrene, its Polymers, Copolymers and Derivatives”, Reinhold (1952)
Non-Patent Document 3: Journal of Applied Polymer Science, Vol. 40, p:41 (1990)